Although myocardial contrast echocardiography (MCE) has provided important insights into coronary pathophysiology, its use in humans has been limited by the need for intra-arterial (direct coronary artery or aortic root) or left heart injections. Recent advances in microbubble technology (development of high molecular weight, non-diffusible gases), novel imaging strategies (harmonic imaging which allows imaging of signals specifically emanating from bubbles as they oscillate at their reonating frequency), and the ability to destroy bubbles with ultrasound and measure their tissue reappearance rate during a constant infusion, have made the evaluation and quantification of myocardial perfusion possible from venous injections. Expanding on our previous work based on arterial and left heart injections of microbubbles in animal models and humans, and adding newly developed canine models of chronic ischemic left ventricular dysfunction and cardiac transplant rejection, we wish to fulfill the following 5 aims in our competitive renewal: 1) Assessing myocardial perfusion during acute myocardial infarction in canine models and humans. 2) Quantification of myocardial blood flow and myocardial blood volume in-vivo in acute and chronic canine models of ischemia. 3) Determining abnormal coronary physiology in the presence of coronary stenoses in acute and chronic canine models of coronary artery disease and in humans. 4) Assessing myocardial viability in chronic coronary artery disease in canine models and humans. 5) Assessing coronary flow reserve in a canine model of cardiac transplant rejection. Hypotheses will be tested in canine models followed by human studies. The impact of the research could be very substantial. By the year 2020, coronary artery disease will be the leading cause of mortality in the world (having surpasses infectious diseases). Secondary prevention or clinical management is based on early detection of disease. Echocardiography is a ubiquitous and relatively inexpensive technology which is used routinely to assess cardiac structures and function. Microbubbles used for MCE remain entirely within the intravascular space and mimic red blood cell rheology in the coronary microcirculation. The basis for our research will be the quantification of myocardial perfusion during continuous infusion of microbubbles, which is discussed in detail in aim #2, and will allow not only the assessment of regional transmural myocardial perfusion but also the transmural distribution of perfusion, as well as its cyclic changes during the cardia cycle. The ultimate goal of this proposal is to quantify myocardial perfusion in humans using venous injections of microbubbles during simultaneously performed echocardiography. Since many disease processes affect the coronary microcirculation, MCE could become a valuable clinical and research tool to study these processes. The knowledge gained from these studies could also be applied to other tissues and organs accessible to ultrasound.